Process for preparing R-1234yf by base mediated dehydrohalogenation

ABSTRACT

The invention relates to a process for preparing 2,3,3,3-tetrafluoropropene (CF 3 CF═CH 2 ), performed using the steps of dehydrohalogenating 1,1,1,2,2-pentafluoropropane (CH 3 CF 2 CF 3 , HFC-245ca) 1,1,1,2-tetrafluoro-2-chloropropane, 1,1,1,2,3-pentafluoropropane (CH 2 FCHFCF 3 , HFC-245eb) and/or 1,1,1,2-tetrafluoro-3-chloropropane in the presence of a base, and converting a trifluorodichloropropane or a difluorotrichloropropane or a fluorotetrachloropropane to CH 3 CF 2 CF 3 , 1,1,1,2-tetrafluoro-2-chloropropane, CH 2 FCHFCF 3 , and/or 1,1,1,2-tetrafluoro-3-chloropropane.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 13/854,726 filed 1 Apr. 2013, which is a divisional of issued U.S. patent application Ser. No. 12/448,324 filed 4 Feb. 2010, now U.S. Pat. No. 8,410,325 issued 2 Apr. 2013, which is the U.S. National Phase entry of International Patent Application Serial No. PCT/GB2007/004840 filed 18 Dec. 2007.

BACKGROUND OF THE INVENTION

The present invention relates to a process for preparing (hydro)fluoroalkenes and particularly to a process for preparing C₃₋₇ (hydro)fluoroalkenes by the dehydrohalogenation of a hydro(halo)fluoroalkane.

The known processes for preparing (hydro)fluoroalkenes typically suffer from disadvantages such as low yields, and/or the handling of toxic and/or expensive reagents, and/or the use of extreme conditions, and/or the production of toxic by-products. This is exemplified by considering the known methods for producing C₃₋₇ (hydro)fluoroalkenes such as 2,3,3,3-tetrafluoropropene. Methods for the preparation of 2,3,3,3-tetrafluoropropene have been described in, for example, Journal of Fluorine Chemistry (82), 1997, 171-174. In this paper, 2,3,3,3-tetrafluoropropene is prepared by the reaction of sulphur tetrafluoride with trifluoroacetylacetone. However, this method is only of academic interest because of the hazards involved in handling the reagents and their expense. Another method for the preparation of 2,3,3,3-tetrafluoropropene is described in U.S. Pat. No. 2,931,840. In this case, pyrolysis of C₁ chlorofluorocarbons with or without tetrafluoroethylene was purported to yield 2,3,3,3-tetrafluoropropene. However, the yields described were very low and again it was necessary to handle hazardous chemicals under extreme conditions. It would also be expected that such a process would produce a variety of very toxic by-products. In addition to addressing the disadvantages of the known methods, it would be desirable to provide new methods for the preparation of (hydro)fluoroalkenes that use only readily available feedstocks.

The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

SUMMARY OF THE INVENTION

The present invention addresses the foregoing deficiencies of the known routes for preparing (hydro)fluoroalkenes by providing a process for preparing 2,3,3,3-tetrafluoropropene (CF₃CF═CH₂), which process comprises dehydrohalogenating 1,1,1,2,2-pentafluoropropane (CH₃CF₂CF₃, HFC-245ca) 1,1,1,2-tetrafluoro-2-chloropropane, 1,1,1,2,3-pentafluoropropane (CH₂FCHFCF₃, HFC-245eb) and/or 1,1,1,2-tetrafluoro-3-chloropropane in the presence of a base, the process further comprising the step of converting a trifluorodichloropropane or a difluorotrichloropropane or a fluorotetrachloropropane to CH₃CF₂CF₃, 1,1,1,2-tetrafluoro-2-chloropropane, CH₂FCHFCF₃, and/or 1,1,1,2-tetrafluoro-3-chloropropane. Unless otherwise indicated, this will be referred to hereinafter as the process of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This base-mediated dehydrohalogenation process comprises contacting the hydro(halo)fluoroalkane with base such as a metal hydroxide or amide (preferably a basic metal hydroxide or amide, e.g. an alkali or alkaline earth metal hydroxide or amide).

Unless otherwise stated, as used herein, a (hydro)fluoroalkene is a linear or branched alkene in which at least one of the hydrogen atoms has been replaced by fluorine. For the avoidance of doubt, the compounds of formula CF₃CF═CHX, CHX₂CX═CX₂, CX₃CX₂CX₂═CX₂, (CX₃)(CX₃)C═CX₂, CFX═CXCX₂CX₂CX₃, CH₂═CXCX₂CX₂CX₃, CX₃CF═CXCX₂CX₃, CX₃CX═CXCX₂CX₂H(CX₃)(CX₃)CXCX═CX₂, (CX₃CX₂)(CX₃)C═CX₂, (CX₃)(CX₃)C═CXCX₃, CX₂═CX(CX₂)₃CX₃, CX₃CX═CX(CX₂)₂CX₃, CX₃CX₂CX═CXCX₂CX₃(CX₃)(CX₃)CXCF═CXCX₃ and (CX₃)(CX₃)CXCX═CFCX₃ herein defined are (hydro)fluoroalkenes.

Unless otherwise stated, as used herein, a hydro(halo)fluoroalkane is a linear or branched alkane in which at least one but not all hydrogen atoms have been replaced by a fluorine atom and optionally at least one hydrogen atom has been replaced by a halogen selected from chlorine, bromine and iodine. Thus, hydro(halo)fluoroalkanes contain at least one hydrogen, at least one fluorine and optionally at least one halogen selected from chlorine, bromine and iodine. In other words, the definition of a hydro(halo)fluoroalkane includes a hydrofluoroalkane, i.e., an alkane in which at least one but not all of the hydrogen atoms have been replaced by fluorine. For example, the compounds of formula CF₃CFYCH₂X, CF₃CFHCYHX, CHX₂CXYCX₂H and CHX₂CXHCX₂Y herein defined are hydro(halo)fluoroalkanes (hydro(halo)fluoropropanes).

Unless otherwise stated, as used herein, any reference to a (C₃₋₇)(hydro)fluoroalkene, hydrofluoroalkane or hydro(halo)fluoroalkane refers to a linear or branched (hydro)fluoroalkene, hydrofluoroalkane or hydro(halo)fluoroalkane having from 3 to 7 carbon atoms, i.e. hydro(halo)fluoro-propane, butane, pentane, hexane or heptane or a (hydro)fluoro-propene, butene, pentene, hexene or heptene.

The (hydro)fluoroalkenes produced by the process of the invention contain a double bond and may thus exist as E (entgegen) and Z (zusammen) geometric isomers about each individual double bond. All such isomers and mixtures thereof are included within the scope of the invention.

Unless otherwise stated, as used herein, by the term “dehydrohalogenation” (or dehydrohalogenating), we refer to the removal of hydrogen halide (e.g. HF, HCl, HBr or HI), for example from a hydro(halo)fluoroalkane. Thus the term “dehydrohalogenation” includes “dehydrofluorination”, “dehydrochlorination”, “dehydrobromination” and “dehydroiodination” of a hydro(halo)fluoroalkane.

Unless otherwise stated, as used herein, by the term “alkali metal hydroxide”, we refer to a compound or mixture of compounds selected from lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide and caesium hydroxide. Similarly, by the term “alkali metal amide”, we refer to a compound or mixture of compounds selected from lithium amide, sodium amide, potassium amide, rubidium amide and caesium amide.

Unless otherwise stated, as used herein, by the term “alkaline earth metal hydroxide”, we refer to a compound or mixture of compounds selected from beryllium hydroxide, magnesium hydroxide, calcium hydroxide, strontium hydroxide and barium hydroxide. Similarly, by the term “alkaline earth metal amide”, we refer to a compound or mixture of compounds selected from beryllium amide, magnesium amide, calcium amide, strontium amide and barium amide.

The process of the invention can be carried out in any suitable apparatus, such as a static mixer, a stirred tank reactor or a stirred vapor-liquid disengagement vessel. The process may be carried out batch-wise or continuously. Either the batch-wise process or the continuous process may be carried out in a “one-pot” fashion, or using two or more discrete reaction zones and/or reaction vessels.

Typically, the process of the invention is conducted at a temperature of from −50 to 300° C. Preferably, the process is conducted at a temperature of from 20 to 250° C., for example from 50 to 200° C.

The process of the invention may be conducted at a pressure of from 0 to 30 bara.

The reaction time for the process of the invention may vary over a wide range. However, the reaction time will typically be in the region of from 0.01 to 100 hours, such as from 0.1 to 50 hours, e.g. from 1 to 20 hours.

Of course, the skilled person will appreciate that the preferred conditions (e.g. temperature, pressure and reaction time) for conducting the process of the invention may vary depending on a number of factors such as the hydro(halo)fluoroalkane being dehydrohalogenated, the base being employed, and/or the presence of a catalyst etc.

The process of the invention may be carried out in the presence or absence of a solvent. If no solvent is used, the hydro(halo)fluoroalkane may be passed into or over molten base or hot base, for example in a tubular reactor. If a solvent is used, in some embodiments a preferred solvent is water, although many other solvents may be used. In some embodiments solvents such as alcohols (e.g. propan-1-ol), diols (e.g. ethylene glycol) and polyols such as polyethylene glycol (e.g. PEG200 or PEG300) may be preferred. These solvents can be used alone or in combination. In further embodiments, solvents from the class known as polar aprotic solvents may be preferred. Examples of such polar aprotic solvents include diglyme, sulfolane, dimethylformamide (DMF), dioxane, acetonitrile, hexamethylphosphoramide (HMPA), dimethyl sulphoxide (DMSO) and N-methylpyrrolidone (NMP). The boiling point of the solvent is preferably such that it does not generate excessive pressure under reaction conditions.

A preferred base is an alkali metal hydroxide selected from the group consisting of lithium hydroxide, sodium hydroxide and potassium hydroxide, more preferably, sodium hydroxide and potassium hydroxide and most preferably potassium hydroxide.

Another preferred base is an alkaline earth metal hydroxide selected from the group consisting of magnesium hydroxide and calcium hydroxide, more preferably calcium hydroxide.

The base is typically present in an amount of from 1 to 50 weight % based on the total weight of the components which make up the process of the invention. Preferably, the base is present in an amount of from 5 to 30 weight %.

The molar ratio of base to hydro(halo)fluoroalkane is typically from 1:20 to 50:1, preferably from 1:5 to 20:1, for example from 1:2 to 10:1.

As mentioned above, the process of the invention may preferably employ water as the solvent. Thus, the dehydrohalogenation reaction may preferably use an aqueous solution of at least one base, such as an alkali (or alkaline earth) metal hydroxide, without the need for a co-solvent or diluent. However, a co-solvent or diluent can be used for example to modify the system viscosity, to act as a preferred phase for reaction by-products, or to increase thermal mass. Useful co-solvents or diluents include those that are not reactive with or negatively impact the equilibrium or kinetics of the process and include alcohols such as methanol and ethanol; diols such as ethylene glycol; ethers such as diethyl ether, dibutyl ether; esters such as methyl acetate, ethyl acetate and the like; linear, branched and cyclic alkanes such as cyclohexane, methylcyclohexane; fluorinated diluents such as hexafluoroisopropanol, perfluorotetrahydrofuran and perfluorodecalin.

The process of the invention is preferably conducted in the presence of a catalyst. The catalyst is preferably a phase transfer catalyst which facilitates the transfer of ionic compounds into an organic phase from, for example, a water phase. If water is used as a solvent, an aqueous or inorganic phase is present as a consequence of the alkali metal hydroxide and an organic phase is present as a result of the fluorocarbon. The phase transfer catalyst facilitates the reaction of these dissimilar components. While various phase transfer catalysts may function in different ways, their mechanism of action is not determinative of their utility in the present invention provided that they facilitate the dehydrohalogenation reaction. The phase transfer catalyst can be ionic or neutral and is typically selected from the group consisting of crown ethers, onium salts, cryptands and polyalkylene glycols and derivatives thereof (e.g. fluorinated derivatives thereof).

An effective amount of the phase transfer catalyst should be used in order to effect the desired reaction, influence selectivity to the desired products or enhance the yield of one preferred alkene isomer over another, e.g. Z-1225ye over E-1225ye (see below); such an amount can be determined by limited experimentation once the reactants, process conditions and phase transfer catalyst are selected. Typically, the amount of catalyst used relative to the amount of hydro(halo)fluoropropane present is from 0.001 to 20 mol %, such as from 0.01 to 10 mol %, e.g. from 0.05 to 5 mol %.

Crown ethers are cyclic molecules in which ether groups are connected by dimethylene linkages. Crown ethers form a molecular structure that is believed to be capable of receiving or holding the alkali metal ion of the hydroxide and to thereby facilitate the reaction. Particularly useful crown ethers include 18-crown-6 (especially in combination with potassium hydroxide), 15-crown-5 (especially in combination with sodium hydroxide) and 12-crown-4 (especially in combination with lithium hydroxide).

Derivatives of the above crown ethers are also useful, such as dibenzyl-18-crown-6, dicyclohexanyl-18-crown-6, dibenzyl-24-crown-8 and dibenzyl-12-crown-4. Other compounds analogous to the crown ethers and useful for the same purpose are compounds which differ by the replacement of one or more of the oxygen atoms by other kinds of donor atoms, particularly N or S. Fluorinated derivatives of all the above may also be used.

Cryptands are another class of compounds useful in the present invention as phase transfer catalysts. These are three dimensional polymacrocyclic chelating agents that are formed by joining bridgehead structures with chains that contain properly spaced donor atoms. The donor atoms of the bridges may all be O, N, or S, or the compounds may be mixed donor macrocycles in which the bridge strands contain combinations of such donor atoms. Suitable cryptands include bicyclic molecules that result from joining nitrogen bridgeheads with chains of (—OCH₂CH₂—) groups, for example as in [2.2.2]cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, available under the brand names Kryptand 222 and Kryptofix 222).

Onium salts that may be used as catalysts in the base-mediated process of the present invention include quaternary phosphonium salts and quaternary ammonium salts, which may be represented by the formulae R¹R²R³R⁴P⁺Z⁻ and R¹R²R³R⁴N⁺Z⁻, respectively. In these formulae, each of R¹, R², R³ and R⁴ typically represent, independently, a C₁₋₁₀ alkyl group, an aryl group (e.g. phenyl, naphthyl or pyridinyl) or an arylalkyl group (e.g. benzyl or C₁₋₁₀ alkyl-substituted phenyl), and Z⁻ is a halide or other suitable counterion (e.g. hydrogen sulphate).

Specific examples of such phosphonium salts and quaternary ammonium salts include tetramethylammonium chloride, tetramethylammonium bromide, benzyltriethylammonium chloride, methyltrioctylammonium chloride (available commercially under the brands Aliquat 336 and Adogen 464), tetra-n-butylammonium chloride, tetra-n-butylammonium bromide, tetra-n-butylammonium hydrogen sulphate, tetra-n-butylphosphonium chloride, tetraphenylphosphonium bromide, tetraphenylphosphonium chloride, triphenylmethylphosphonium bromide and triphenylmethylphosphonium chloride. Benzyltriethylammonium chloride is preferred for use under strongly basic conditions.

Other useful onium salts include those exhibiting high temperature stabilities (e.g. up to about 200° C.), for example 4-dialkylaminopyridinium salts, tetraphenylarsonium chloride, bis[tris(dimethylamino)phosphine]iminium chloride and tetrakis[tris(dimethylamino)phosphinimino]phosphonium chloride. The latter two compounds are also reported to be stable in the presence of hot, concentrated sodium hydroxide and, therefore, can be particularly useful.

Polyalkylene glycol compounds useful as phase transfer catalysts may be represented by the formula R⁶O(R⁵O)_(m)R⁷ wherein R⁵ is a C₁₋₁₀ alkylene group, each of R⁶ and R⁷ are, independently H, a C₁₋₁₀ alkyl group, an aryl group (e.g. phenyl, naphthyl or pyridinyl) or an arylalkyl group (e.g. benzyl or C₁₋₁₀ alkyl-substituted phenyl), and m is an integer of at least 2. Preferable both R⁶ and R⁷ are the same, for example they may both by H.

Such polyalkylene glycols include diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, hexaethylene glycol, diisopropylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol and tetramethylene glycol, monoalkyl glycol ethers such as monomethyl, monoethyl, monopropyl and monobutyl ethers of such glycols, dialkyl ethers such as tetraethylene glycol dimethyl ether and pentaethylene glycol dimethyl ether, phenyl ethers, benzyl ethers of such glycols, and polyalkylene glycols such as polyethylene glycol (average molecular weight about 300) and polyethylene glycol (average molecular weight about 400) and the dialkyl (e.g. dimethyl, dipropyl, dibutyl)ethers of such polyalkylene glycols.

Combinations of phase transfer catalysts from within one of the groups described above may also be useful as well as combinations or mixtures from more than one group. Crown ethers and quaternary ammonium salts are the currently preferred groups of catalysts, for example 18-crown-6 and its fluorinated derivatives and benzyltriethylammonium chloride.

The process of the invention is used to prepare a compound of formula CF₃CF═CHX or CHX₂CX═CX₂ or a linear or branched C₄₋₇ (hydro)fluoroalkene, wherein each X is, independently, H or F provided that in CHX₂CX═CX₂ at least one X is F.

Preferably, the process is used to prepare a compound of formula CF₃CF═CHX or CHX₂CX═CX₂.

The compound of formula CHX₂CX═CX₂ may be represented by CHX₂CX═CF₂. This compound may be prepared by dehydrohalogenating a compound of formula CHX₂CHXCYF₂ or CHX₂CYXCHF₂. Compounds of the formula CHX₂CX═CF₂ include CHF₂CH═CF₂, CHF₂CF═CF₂, CH₂FCH═CF₂, CH₂FCF═CF₂, CH₃CF═CF₂.

Alternatively, the compound of formula CHX₂CX═CX₂ may not have a terminal ═CF₂ group. Examples of such compounds include CHF₂CH═CHF, CHF₂CF═CHF, CHF₂CF═CH₂.

The compounds of formula CF₃CF═CHX are 2,3,3,3-tetrafluoropropene (CF₃CF═CH₂, HFC-1234yf) and 1,2,3,3,3-pentafluoropropene (CF₃CF═CHF, HFC-1225ye).

The process of the invention may also be used to prepare a linear and/or branched C₄₋₇ (hydro)fluoroalkene.

For example, linear and/or branched (hydro)fluorobut-1-enes or (hydro)fluorobut-2-enes may be prepared by the process of the invention.

Linear (hydro)fluorobut-1-enes may be represented by compounds having the formula CX₃CX₂CX₂═CX₂ wherein X is H or F and at least one X=F. A preferred group of (hydro)fluorobut-1-enes have a boiling point in the range of from 15 to 50° C.

The linear (hydro)fluorobut-1-enes include compounds of the formula CF₃CX₂CX₂═CX₂ wherein X=H or F. Examples of such compounds include CF₃CF₂CF═CF₂, CF₃CF₂CF═CFH, CF₃CHFCF═CF₂, CF₃CF₂CH═CF₂, CF₃CF₂CF═CH₂, CF₃CF₂CH═CHF and CF₃CHFCF═CHF.

Further linear (hydro)fluorobut-1-enes include compounds of the formula CX₃CX₂CX₂═CF₂ wherein X=H or F. Examples of such compounds include CHF₂CF₂CF═CF₂, CH₂FCF₂CF═CF₂, CHF₂CFHCF═CF₂, CH₃CF₂CF═CF₂.

Another group of linear (hydro)fluorobut-1-enes may be represented by the formula CX₃CX₂CX₂═CHF wherein X=H or F. Examples of such compounds include CHF═CFCF₂CHF₂, CHF═CHCHFCF₃, CHF═CHCHFCF₃, CHF═CFCHFCHF₂, CHF═CHCF₂CHF₂ and CHF═CFCH₂CF₃.

Yet another group of linear (hydro)fluorobut-1-enes may be represented by the formula CX₃CX₂CX₂═CH₂ wherein X=H or F, provided that at least one X is F. Examples of such compounds include CH₂═CFCF₂CHF₂, CH₂═CHCF₂CF₃, CH₂═CHCF₂CHF₂— and CH₂═CFCFHCF₃.

Of course, certain linear (hydro)fluorobut-1-enes may fall within more than one of the groups set out above.

Branched (hydro)fluorobut-1-enes (iso-(hydro)fluorobutenes) may be represented by compounds having the formula (CX₃)(CX₃)C═CX₂ wherein X=H or F, provided that at least one X is F. Examples of such compounds include CH₂═C(CHF₂)₂, CHF═C(CF₃)(CH₃), CF₂═C(CHF₂)(CH₃), CF₂═C(CF₃)(CH₃), CH₂═C(CF₃)₂, CH₂═C(CHF₂)(CF₃) and CHF═C(CF₃)₂.

Examples of (hydro)fluorobut-2-enes which may be prepared by the process of the invention include CF₃CF═CFCF₃, CF₃CF═CFCHF₂, CF₃CF═CHCF₃, CF₃CF═CHCH₃, CF₃CF═CFCH₃, CHF₂CF═CHCHF₂, CH₂FCF═CFCHF₂ and CHF₂CF═CFCHF₂.

Linear and/or branched (hydro)fluoropent-1-enes or (hydro)fluoropent-2-enes may be prepared by the process of the invention.

Linear (hydro)fluoropent-1-enes which may be prepared by the process of the invention include compounds of the formula CFX═CXCX₂CX₂CX₃ (wherein X is F or H) and/or compounds of the formula CH₂═CXCX₂CX₂CX₃ (wherein X is F or H, provided that at least one X=H and at least one X=F).

Linear (hydro)fluoropent-2-enes which may be prepared by the process of the invention include compounds of the formula CX₃CF═CXCX₂CX₃ (wherein X is F or H) and/or compounds of the formula CX₃CX═CXCX₂CX₂H (wherein X is F or H, provided that at least one X=F).

Branched (hydro)fluoropent-1-enes may be represented by compounds having the formula (CX₃)(CX₃)CXCX═CX₂ or (CX₃CX₂)(CX₃)C═CX₂ wherein X=H or F, provided that at least one X is F. Branched (hydro)fluoropent-2-enes may be represented by compounds having the formula (CX₃)(CX₃)C═CXCX₃, wherein X=H or F, provided that at least one X is F.

Linear and/or branched (hydro)fluorohex-1-enes or (hydro)fluorohex-2-enes or (hydro)fluorohex-3-enes may be prepared by the process of the invention.

Linear (hydro)fluorohex-1-enes may be represented by the formula CX₂═CX(CX₂)₃CX₃ wherein X=H or F, provided that at least one X is F.

Linear (hydro)fluorohex-2-enes may be represented by the formula CX₃CX═CX(CX₂)₂CX₃ wherein X=H or F, provided that at least one X is F.

Linear (hydro)fluorohex-3-enes may be represented by the formula CX₃CX₂CX═CXCX₂CX₃ wherein X=H or F, provided that at least one X is F. Linear (hydro)fluorohex-3-enes which may be prepared by the process of the invention include compounds of the formula CX₃CX₂CF═CXCX₂CX₃ (wherein X is F or H).

Branched (hydro)fluorohex-2-enes include compounds having the formula (CX₃)(CX₃)CXCF═CXCX₃ wherein X=H or F and/or compounds of the formula (CX₃)(CX₃)CXCX═CFCX₃ wherein X=H or F.

By way of example and for simplicity, unless otherwise stated, the remainder of the description will describe the process of the invention with reference to the preparation of HFC-1234yf and/or HFC-1225ye. Of course, the skilled person will understand that the present invention is not limited to the preparation of these compounds. The invention may also be used to prepare other (hydro)fluoropropenes and linear or branched C₄₋₇ (hydro)fluoroalkenes as described above.

HFC-1234yf and 1,3,3,3-tetrafluoropropene (CF₃CH═CHF, HFC-1234ze) may be together prepared by the process of the invention. Alternatively, HFC-1234yf and HFC-1225ye may be separately prepared by the process of the invention.

HFC-1234yf may be prepared by dehydrohalogenating a compound of formula CF₃CFYCH₃ or CF₃CFHCYH₂, where Y is F, Cl, Br, or I, preferably, F or Cl.

HFC-1234yf may be prepared by a process comprising the dehydrofluorination of 1,1,1,2,2-pentafluoropropane (CH₃CF₂CF₃, HFC-245ca) or 1,1,1,2,3-pentafluoropropane (CH₂FCHFCF₃, HFC-245eb). Both HFC-245ca and HFC-245eb may be obtained from Apollo Chemicals Limited. Alternatively, 1,1,1,2,2-pentafluoropropane, for example, may be prepared by fluorinating one or more of a large number of hydrochlorofluoropropanes including tetrafluorochloropropanes such as 1,1,1,2-tetrafluoro-2-chloropropane and 1,1,2,2-tetrafluoro-1-chloropropane, trifluorodichloropropanes such as 1,1,1-trifluoro-2,2-dichloropropane, 1,1,2-trifluoro-1,2-dichloropropane and 1,2,2-trifluoro-1,1-dichloropropane, difluorotrichloropropanes such as 2,2-difluoro-1,1,1-trichloropropane, 1,2-difluoro-1,1,2-trichloropropane and 1,1-difluoro-1,2,2-trichloropropane and fluorotetrachloropropanes such as 1-fluoro-1,1,2,2-tetrachloropropane and 2-fluoro-1,1,1,2-tetrachloropropane. 1,1,1,2,2-pentafluoropropane (and thus ultimately HFC-1234yf) may also be prepared starting from 1,1,1,2,2-pentachloropropane. In any of the above hydrohalo(fluoro)propane precursors to 1,1,1,2,2-pentafluoropropane, one or more of the chlorine substituents may be replaced by bromine or iodine.

Preferred hydro(halo)fluoropropanes for preparing HFC-1234yf include 1,1,1,2,2-pentafluoropropane, 1,1,1,2,3-pentafluoropropane, 1,1,1,2-tetrafluoro-2-chloropropane and 1,1,1-trifluoro-2,2-dichloropropane. It will be understood by the skilled person that 1,1,1-trifluoro-2,2-dichloropropane may be fluorinated to give 1,1,1,2-tetrafluoro-2-chloropropane and/or 1,1,1,2,2-pentafluoropropane. 1,1,1,2-tetrafluoro-2-chloropropane may also be fluorinated to produce 1,1,1,2,2-pentafluoropropane, which may then be dehydrofluorinated to give HFC-1234yf.

Alternatively, 1,1,1,2-tetrafluoro-2-chloropropane may be dehydrochlorinated to give HFC-1234yf.

The reaction pathways described above for producing HFC-1234yf from 1,1,1,2,2-pentafluoropropane, 1,1,1,2-tetrafluoro-2-chloropropane and 1,1,1-trifluoro-2,2-dichloropropane are illustrated below.

In a further embodiment, HFC-1234yf may be prepared starting from 1,1,1-trifluoro-2,3-dichloropropane, which can readily be prepared by chlorinating 1,1,1-trifluoromethylpropene. It is believed that there are two principal routes to HFC-1234yf from 1,1,1-trifluoro-2,3-dichloropropane, as illustrated below.

Route B proceeds via the fluorination (e.g. using HF, optionally in the presence of a chromia-containing catalyst) of 1,1,1-trifluoro-2,3-dichloropropane to give 1,1,1,2,3-pentafluoropropane (HFC-245eb) which is then dehydrofluorinated to give HFC-1234yf. Alternatively, 1,1,1,2-tetrafluoro-3-chloropropane (which is an intermediate in the fluorination of 1,1,1-trifluoro-2,3-dichloropropane to HFC-245eb) may be dehydrochlorinated to give HFC-1234yf.

Route A proceeds by dehydrochlorination of 1,1,1-trifluoro-2,3-dichloropropane to give 3,3,3-trifluoro-2-chloropropene which is then hydrofluorinated to give 1,1,1,2-tetrafluoro-2-chloropropane. These two steps may be carried out in one pot by contacting 1,1,1-trifluoro-2,3-dichloropropane with HF in the presence of a catalyst. However, it is believed that a two-stage reaction zone may be preferred, the first zone employing a relatively low HF:organics ratio (e.g. from about 1:1 to about 5:1) to promote the dehydrochlorination and the second zone employing a relatively high HF:organics ratio (e.g. from about 5:1 to about 30:1) to promote the hydrofluorination. As described above, 1,1,1,2-tetrafluoro-2-chloropropane may be fluorinated to produce 1,1,1,2,2-pentafluoropropane (e.g. using HF, optionally in the presence of a chromia-containing catalyst), which may then be dehydrofluorinated to give HFC-1234yf. Alternatively, 1,1,1,2-tetrafluoro-2-chloropropane may be directly dehydrochlorinated to give HFC-1234yf.

In summary, HFC-1234yf may be prepared by dehydrohalogenating a compound of formula CF₃CFYCH₃ or CF₃FHCYH₂, e.g. HFC-245ca, HFC-245eb, 1,1,1,2-tetrafluoro-2-chloropropane or 1,1,1,2-tetrafluoro-3-chloropropane.

1,1,1-trifluoro-2,3-dichloropropane is commercially available, but may also be prepared via a synthetic route starting from the cheap feedstocks carbon tetrachloride (CCl4) and ethylene. These two starting materials may be telomerised to produce 1,1,1,3-tetrachloropropane, which may then be fluorinated to produce 1,1,1,3-tetrafluoropropane and/or 1,1,1-trifluoro-3-chloropropane (e.g. using HF, optionally in the presence of a chromia-containing catalyst). Dehydrohalogenation of 1,1,1,3-tetrafluoropropane and/or 1,1,1-trifluoro-3-chloropropane (e.g. using NaOH or KOH) produces 3,3,3-trifluoropropene, which may then be readily chlorinated (e.g. with chlorine) to produce 1,1,1-trifluoro-2,3-dichloropropane. This reaction scheme is summarized below, where X=F or Cl.

As mentioned above, 1,1,1,2,2-pentafluoropropane may be prepared starting from 1,1,1,2,2-pentachloropropane. In this route (see below), 1,1,1,2,2-pentachloropropane is fluorinated (e.g. using HF, optionally in the presence of a chromia-containing catalyst) to produce 1,1,1,2-tetrafluoro-2-chloropropane, which may also be fluorinated to produce 1,1,1,2,2-pentafluoropropane followed by dehydrofluorination to give HFC-1234yf. Alternatively, 1,1,1,2-tetrafluoro-2-chloropropane may be directly dehydrochlorinated to give HFC-1234yf.

1,1,1,2,2-pentachloropropane is a convenient intermediate in a route to HFC-1234yf starting from acetone. In such a synthetic route, acetone may be chlorinated (for example using chlorine over a chromia catalyst) to produce 1,1,1-trichloroacetone, which may be further chlorinated (for example using PCl₅—see Advanced Organic Chemistry (Ed M B Smith and J March), Fifth Edition, page 1195) to produce 1,1,1,2,2-pentachloropropane, as illustrated below.

HFC-1225ye may be prepared by dehydrohalogenating a compound of formula CF₃CFYCH₂F or CF₃CFHCYFH, where Y is F, Cl, Br, or I, preferably F or Cl, most preferably F. Thus, HFC-1225ye is currently most preferably prepared by the dehydrofluorination of CF₃CFHCF₂H (HFC-236ea) or CF₃CF₂CH₂F (HFC-236cb), as illustrated below.

HFC-1225ye exists as two geometric isomers, as illustrated below.

Both HFC-236ea and HFC-236cb may be obtained from Apollo Chemicals Ltd. Alternatively, HFC-236ea, for example, can be conveniently prepared by hydrogenating hexafluoropropylene as follows. CF₃CF═CF₂+H₂→CF₃CHFCHF₂

The (hydro)fluoroalkenes which may be prepared by the present invention have numerous utilities, for example as refrigerants, monomers, feedstocks and solvents.

The invention will now be illustrated, but not limited, by the following examples. The invention is defined by the claims following the examples.

EXAMPLES Examples 1-7

Solvent (10 g), alkali metal hydroxide (10 g) and (where used) catalyst (0.25 g) were charged to a 50-ml Hastalloy C reactor vessel equipped with temperature and pressure indicators and a cruciform stirrer. The vessel was sealed and pressure tested with nitrogen. A feed of either CF₃CFHCF₂H (HFC-236ea) or CF₃CF₂CH₂F (HFC-236cb) (10-15 g, 97%) was then charged from a small feed bomb and the contents heated at 150° C. for the times specified in Table 1 with stirring. At the end of the experiment, the volatile products and any unconverted feeds were recovered by distillation for analysis by GC-MS. The results are illustrated in Table 1.

TABLE 1 236ea/cb → 1225ye HFC- Z- E- 122 Expt 236 Cata- Time Conver- 1225ye 1225ye 5zc No isomer Solvent Base lyst (hrs) sion (%) (%) (%) (%) 1 ea water KOH — 3.5 57.8 46.7 5.7 0 2 ea water NaOH — 4.5 20.8 14.9 1.7 0 3 ea water KOH — 19 21.4 15.9 1.3 0 4 ea water KOH 18- 2.5 57.5 49.3 4.1 0 crown- 6 5 ea PEG300 KOH — 3 50.4 41 2.8 0 6 cb water KOH — 19 1.3 0.52 0 N/A 7 cb water KOH — 22 1.5 0.42 0 N/A

Examples 8-22

Base, solvent and catalyst, where used, were charged to the reaction vessel. The reaction vessel was sealed, pressure tested and evacuated. A pre-weighed amount of the organic feed was then transferred to the reactor. The reactor and its contents were then weighed before being heated from room temperature to 150° C. over 45 minutes with stirring. This temperature was maintained with stirring at 1500 rpm for 6 hours. After this period the reactor and its contents were cooled to 10° C. over 30 minutes and the rate of stirring reduced to 200 rpm.

The next day the reactor and its contents were re-weighed and then the reactor re-heated to 50° C. ready for product recovery. The products and any un-reacted starting materials were recovered by distillation into a pre-weighed and evacuated chilled (−78° C.) sample bomb. The weight of the recovered products was determined before they were analyzed by GC-MS. The GC-MS was calibrated using feed and product samples where available. Unknowns were quantified using average relative response factors. The results are presented in Tables 2-4.

TABLE 2 236ea → 1225ye Selectiv- Reactor Mass 236ea ity to Experiment: Base Solvent Catalyst weight recovered conver- Z&E- Ratio Substrate- Mass Mass Mass Start End products sion 1225ye Z:E (g) Type (g) Type (g) Type (g) (g) (g) (g) (%) (%) isomers 8: 236ea- KOH 5 Ethylene 10 NR₄X 0.25 2963.5 2963.1 8.39 60.1 96.4 13.8 13.72 glycol Water 10 9: 236ea- KOH 5 Ethylene 10 None — 2963.6 2963.5 9.03 60.2 96.4 13.6 14.29 glycol Water 10 10: 236ea- KOH 5 Propan-1-ol 25 None — 2969.3 2969.2 7.47 76.1 97.4 23.6 14.2 11: 236ea- KOH 5 Propan-1-ol 25 NR₄X 0.25 2969.2 2961.1 8 85.8 92.6 18.0 14.3 12: 236ea- KOH 5 PEG 200 25 None — 2968.8 2964.1 4.5 82.6 89.5 17.6 14.0 13: 236ea- KOH 5 NMP 25 None — 2959.8 2959.6 8.9 97.5 97.0 23.9 14.5 14: 236ea- KOH 5 Water 12.5 NR₄X 0.25 2959.3 2959.2 11.9 69.0 97.7 17.7 14.0 Perfluoro- 12.5 decalin 15: 236ea- Ca 5.2 NMP 25.4 None — 2959.4 2959.7 9.0 96.0 92.5 10.2 14.6 (OH)₂ NR₄X = Benzyltriethylammonium chloride; NMP = N-methyl pyrrolidone; PEG = Polyethylene glycol MW200.

TABLE 3 245ca → 1234yf Reactor Mass Experiment: Base Solvent Catalyst weight recovered 245ca Selectivity Substrate - Mass Mass Mass Start End products conversion to 1234yf (g) Type (g) Type (g) Type (g) (g) (g) (g) (%) (%) 16: 245ca- KOH 5.2 Propan-1- 25.3 None — 2965.5 2965.5 7.0 18.2 91.0 10.7 ol 17: 245ca- KOH 5.2 Propan-1- 24.9 18-Crown-6 0.25 2967.1 2966.9 7.7 19.4 94.7 11.4 ol 18: 245ca- KOH 5 Propan-1- 25 NR₄X 0.25 2965.8 2965.8 7.1 17.0 94.7 11.1 ol 19: 245ca- KOH 5 NMP 25 None — 2952.8 2952.4 5.9 22.8 93.4 8.73 NR₄X = Benzyltriethylammonium chloride; NMP = N-methyl pyrrolidone.

TABLE 4 245eb → 1234yf Reactor Mass Experiment: Base Solvent Catalyst weight recovered 245eb Selectivity Substrate - Mass Mass Mass Start End products conversion to 1234yf (g) Type (g) Type (g) Type (g) (g) (g) (g) (%) (%) 20: 245eb- KOH 5.1 NMP 26.8 None — 2957.9 2957.8 9.6 100 99.6 11.8 21: 245eb- Ca(OH)₂ 5.0 NMP 26.2 None — 2954.0 2954.1 6.5 100 99.6 8.4 NMP = N-methyl pyrrolidone. 

We claim:
 1. A process for preparing 2,3,3,3-tetrafluoropropene (CF₃CF═CH₂), which process comprises dehydrohalogenating a compound of formula CF₃CFYCH₃ or CF₃CFHCYH₂, wherein Y is F, Cl, Br, or I, in the presence of a base selected from an alkali metal hydroxide and an alkaline earth metal hydroxide, wherein the process is carried out in the absence of a phase transfer catalyst; and wherein the process is carried out in a polar aprotic solvent.
 2. A process according to claim 1 carried out at a temperature of from −50 to 300° C. and a pressure of from 0 to 30 bara.
 3. A process according to claim 1 wherein the alkali metal hydroxide is selected from sodium hydroxide and potassium hydroxide.
 4. A process according to claim 1 wherein the alkaline earth metal hydroxide is calcium hydroxide.
 5. A process according to claim 1 wherein the polar aprotic solvent is selected from the group consisting of: diglyme, dimethylformamide (DMF), dioxane, acetonitrile, hexamethylphosphoramide (HMPA) and N-methylpyrrolidone (NMP).
 6. A process according to claim 1 wherein CF₃CF═CH₂ is prepared by dehydrofluorinating 1,1,1,2,2-pentafluoropropane (CH₃CF₂CF₃, HFC-245ca), dehydrochlorinating 1,1,1,2-tetrafluoro-2-chloropropane, dehydrofluorinating 1,1,1,2,3-pentafluoropropane (CH₂FCHFCF₃, HFC-245eb) and/or dehydrochlorinating 1,1,1,2-tetrafluoro-3-chloropropane.
 7. A process according to claim 6 comprising the step of converting a trifluorodichloropropane or a difluorotrichloropropane or a fluorotetrachloropropane to CH₃CF₂CF₃, 1,1,1,2-tetrafluoro-2-chloropropane, CH₂FCHFCF₃, and/or 1,1,1,2-tetrafluoro-3-chloropropane.
 8. A process according to claim 7 wherein the trifluorodichloropropane is selected from 1,1,1-trifluoro-2,2-dichloropropane (CF₃CCl₂CH₃) and 1,1,1-trifluoro-2,3-dichloropropane (CF₃CHClCH₂Cl).
 9. A process according to claim 8 comprising the step of chlorinating 3,3,3-trifluoropropene to produce CF₃CHClCH₂Cl.
 10. A process according to claim 9 comprising the step of dehydrofluorinating 1,1,1,3-tetrafluoropropane to produce 3,3,3-trifluoropropene.
 11. A process according to claim 10 comprising the step of fluorinating 1,1,1,3-tetrachloropropane to produce 1,1,1,3-tetrafluoropropane.
 12. A process according to claim 11 comprising the step of telomerising carbon tetrachloride and ethylene to produce 1,1,1,3-tetrachloropropane.
 13. A process according to claim 8 comprising the step of fluorinating 1,1,1,2,2-pentachloropropane to produce CF₃CCl₂CH₃.
 14. A process according to claim 13 comprising the step of chlorinating 1,1,1-trichloroacetone (CCl₃C(O)CH₃) to produce 1,1,1,2,2-pentachloropropane.
 15. A process according to claim 14 comprising the step of chlorinating acetone (CH₃C(O)CH₃) to produce 1,1,1-trichloroacetone (CCl₃C(O)CH₃). 